Which one of the following does a TV remote control unit use to operate a TV set ?

Ordinary TV remote controls use infrared light (a form of electromagnetic “light” waves) emitted by an LED that pulses coded signals the television’s IR receiver decodes; IR is the dominant home-theatre remote technology and requires line-of-sight and produces very short, coded light pulses. A digital camera can detect this pulsing IR even when it’s invisible to the naked eye, demonstrating the light-based nature of the signal [1]. Sound waves are mechanical vibrations in a medium and are not used in modern TV remotes for coded, line-of-sight control, so option (2) is incorrect [1]. Thus the correct choice is Light waves (infrared).

Sources

1. [1] https://www.exploratorium.edu/snacks/infrared-remote
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Why Do Sound Waves Travel Faster In A Denser Medium Whereas Light Travels Slower? > p. 64